7368
J . Phys. Chem. 1991,95, 7368-7372
interaction parameters, which yield information concerning the molecular geometry. By analyzing the multiplet sideband structure of 31P and the central transition of 27AlNMR, it was found that the aluminum chloride with TMP adducts exists in two dominant forms, which are in an equilibrium similar to that found for AlC13 and P(C2HS)3adducts in benzene solution.6 When T M P is evacuated the 1: 1 (A1:P) type complex is dominant (aA1 = 1 11.5 ppm, bP = -42.5 ppm), in which aluminum is four-coordinated and presumably is present as (CH3),P-AIC13. When the T M P loading is high, a large portion of the complex appears with an AI-P ratio of 1:2. This phase (bA1 = 54.3 ppm, b p = 4 0 . 4 ppm) was found to have five-coordinated aluminum with a proposed structure of (CH3)3P-AIC13-P(CH3)3. These complexes have been observed both in the presence and the absence of the zeolite support. Higher TMP coordination is also possible; however, such complexes were not discernible in the present experiment. The AI-P internuclear distances calculated from the dipolar coupling constants determined from the direct sideband analysis are 2.58 and 2.96 A, respectively, for the tetrahedral and the trigonalbipyramidal complex. Further structural identification comes from a comparison of the AI-P distances measured from MAS-NMR spinning sideband intensity analysis with those given by vibrational
spectra and powder X-ray pattern analysis for a series of similar complexes.25 The quadrupolar coupling constants for aluminum in these two types of molecules are 370 kHz and 4.7 MHz and the asymmetry factors are 0.0 and 0.2, respectively, which reveals the different nature of the aluminum bonding in the two species. By examining N M R data for samples prepared using different methods, the mechanisms governing the reaction can be inferred. It is proposed that the initial step in the formation of the adducts is the adsorption of P(CH3)3on solid aluminum chloride and the reaction, which is promoted by the excessive basic TMP molecule, takes place outside the zeolite crystallites. A role of the zeolite is therefore to dilute the AIC13 and facilitate the reaction.
Acknowledgment. This research was supported by the State of Texas Advanced Technology Program. A. de Mallmann acknowledges CNRS (France) and NSF for a grant obtained through the CNRS/NSF exchange program. We are also indebted to Professors T. Hughbanh and A. Clearfield for the use of equipment. (25) Beattie, 1. R.; Ozin, G. A.; Blayden, H. E. J . Chem. Soc. A 1969, 2535.
Study on d State of Piatlnum in Pt/Si02 and Na/Pt/SiO, Catalysts under C=C Hydrogenation Conditions by X-ray Absorption Near-Edge Structure Spectroscopy Hideaki Yoshitakef and Yasuhiro Iwasawa* Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan (Received: October 23, 1990) The change in the d-electron density of platinum during D2 + CH2=CHX reactions on Pt/Si02 and Na/Pt/Si02 catalysts and its influence on the catalysis were studied by X-ray absorption near-edge structure (XANES) spectroscopy, kinetics and FT-IR.It was demonstrated from the change of the white lines in XANES spectra at Pt L2and L3 edges that CHZaCHX (X = H, CH3, COCH3, CF,, and CN) is adsorbed on the Pt surface and extracts the electrons of the d state. Hence, the deuterogenation rate is reduced as the value of Hammett's up increases. The linear free energy relationship between the reaction rate and upwas observed for the deuterogenation of CHZ=CHX. The rate of ethene deuterogenation was promoted by NazO addition. The electron density of unoccupied d states of Pt under vacuum decreased by Na20 addition, indicating the electron donation from Na20to Pt particles. However, most of these additional electrons were observed to move to ethene under reaction conditions. The acceptor of the electrons was suggested to be di-u-ethene by the shift of v(C-H). The kinetic parameters are discussed in relation to the change in the d state of Pt as a function of up and Na quantity.
The promoting effects of supports or additives on the catalysis of group 8 metals have been demonstrated to be based on many factors, but the electronic modification of metals may be an indispensable and universal aspect brought by promoters. Thus, the electronic properties of the metal ensembles are often key ingredients for the elucidation of the genesis or the mechanism of noble-metal catalysis. The change in the density of d electrons should be essential for metal catalysis since the adsorption or activation of reactants arises mainly from the interaction of d electrons with frontier orbitals of reactant molecules. The white lines emerging at X-ray adsorption edges of platinum correspond to the electronic transition from 2p core electrons to unoccupied 5d state of Pt by photon absorption. The L2 edge arises from the 2pIl2states, and the L3 edge arises from the 2pY2states. It has been clarified that the intensity of the white lines reflects the electronic characteristics of platinum catalyst, and this relation has been applied to the supported catalysts under vacuum14 or Address correspondence to this author. 'Present address: Department of Energy Engineering, Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240, Japan.
0022-3654/9 1/2095-7368$02.50/0
ambient oxygen, hydrogen, or carbon Further, the degree of unoccupied d states of Pt has quantitatively been estimated by the intensity of the white lines* in investigating I metal-support It has been reported that catalytic reactions often originate from the change of the structures and electronic states of metal sites, (1) Lewis, P. H. J . Phys. Chem. 1963, 67, 2151. (2) Horsley, J . A.; Lytle, F. W. ACS Symp. Ser. 1986, 298, 10. (3) Lytle, F. W. J . Curul. 1976,13, 376.
(4) Gallezot, P.; Weber. R.; Dalla Betta, R. A,; Boudart, M. Z. Nulurforsch. 1979, 34A, 40. (5) Lytle, F. W.; Wei. P. S.P.; Greegor. R. B.; Via, G. H.; Sinfelt, J. H. J . Chem. Phys. 1979, 70,4849. (6) Fukushima, T.; Katzer, J. R.; Sayers, D. E.; Cook, J. Proceedings, 9th International Congress on Catalysis; 1988; Vol. 1, p 79. (7) McHugh, B. J.; Lawn, G.; Haller, G. L. Unpublished results. (8) Mansour, A. N.; Cook, J. W., Jr.; Sayers, D. E. J . Phys. Chem. 1984, 88, 330. (9) Short, D. R.; Mansour, A. N.; Cook, J. W., Jr.; Sayers, D. E.; Katzer, J . R. J. Cutul. 1983,82, 299. (IO) Mansour, A. N.; Cook, J. W., Jr.; Sayers, D. E.; Emrich, R. J.; Katzer, J. R. J . Cutul. 1984, 89, 462. ( I 1) Resasco, D. E.; Weber, R. S.: Sakellson. S.: McMillan. M.: Haller. G. L.J. Phys. Chem. 1988, 92, 189.
0 199 1 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 1369
XANES of Pt Catalyst in Reaction Conditions which is hardly pictured from static experiment^.'^-^' Thus, in situ studies on the working catalyst are desirable. We have investigated the relation between the electronic state of the catalyst metal particle and the elementary step of the catalytic C - C hydrogenation by monitoring the electron density of d states of supported Pt catalysts under the reaction conditions. Moleculemolecule interaction through the metal often plays an important role at the key step of the reaction to change the rate or the selectivity. The introduction of substituents to C - C compounds in this study should belong to this case. The electron donation by the metal additives which compose the catalyst surface could also affect the catalysis. Both cases are promotion effects as Thomson pointed 0 ~ t . IThe ~ measurement of the platinum d state monitors the electron media between molecule and molecule or between molecule and alkali metal in a working state of the catalyst.
Methods Pt/Si02 and Na/Pt/Si02 catalysts were prepared by an impregnation method with an aqueous solution of H2PtC16.6H20 (Soekawa Chemical Co., Ltd., research grade) or a mixture of H2PtC16.6H20 and Na2C03 (Wako Pure Chemical Co., Ltd., 99.7%). Si02was commercially obtained from Nippon Aerosil ox-50, surface area 50 m2.g-'. The sample was allowed to stand for 12 h and then dried at 393 K, followed by calcination at 773 K. The loading of platinum metal was 2.4%. The samples thus obtained were placed in a U-shaped tube in a closed circulating system and oxidized with O2at 673 K and reduced at 473 K in situ before catalytic reactions. Five kinds of Na/Pt/Si02 with different amounts of Na were prepared: Na/(Na + Pt) = 0.16, 0.43,0.60,0.70, and 0.86 in molar ratio, which correspond to the atomic ratios, Na/Pt, 0.19,0.75, 1.5, 2.3, and 6.1, respectively. X-ray absorption near-edge structure (XANES) spectra were measured in the transmission mode at BL-7C of Photon Factory, National Laboratory for High Energy Physics (KEK-PF, Proposal 89-007). The samples were prepared in a closed circulating system and transferred to an in situ X-ray absorption cell. CH2=CH2, C H 2 C H C H 3 (Takachiho Trading Co., Ltd., 99.9%), CH2=CHCN, CH2=CHCOCH3 (Tokyo Kasei Co., Ltd.), and CH2=CHCF3 (Japan Halon, 99.9%) were purified by freeze-thaw cycles before use. Deuterium (Takachiho Trading Co., Ltd., research grade) was purified through a 5A molecular sieve trap at 77 K. D2 CH2 CH3 > H > COCH3 > CN. The order is usually explained quantitatively by Hammett's parameter up of the substituents as plotted in Figure 2, which exhibits a linearity (16) Brown,
M.;Peierls, R. E.;Stern, E. A. Phys. Rev. B 1977, IS, 738.
7370 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 TABLE I: fr for 2.4% Pt/SiO, CH24HX none CH24HCHl CH,=CH, CH2-C HCOCH3 CH,=CHCFl CH,=CHCN
under 1.3 kPa of CH,=CHX UP
fd
-0.17 0.00 0.50 0.54 0.66
0.058 0.076 0.133 0.190 0.074 0.212
TABLE III: Kinetic Parameters for Ethane Formation and Amount of Adsorbates during the ReactionO
0.018 0.075 0.132 0.016 0.154
0.00 0.75 1.5 2.3 6.I
"Afd = fd(CH2=CHX)-fd(none).
TABLE 11: Dependence of f r and hT on the Particle Size in Pt/SiO, loading, % HIM fd hT 0.9 0.67 0.17 0.35 0.31 0.014
2.4 4.7
0.06 0.02
Yoshitake and Iwasawa
1.8 X lo-' 8.8 X lo4 8.2 X lo-' 7.6x 104 6.8 X IO4
33 41 41 41 41
IO-,
2.3 X 1.7 X 1.8X 3.8 x 7.5 X
IO-3
10-3 10-3 10-3
0.94 0.87 0.86 0.83 0.71
" E.,, activation energy. K E and kD2,constants in the rate expression r = kD2P02/( 1 + KEPE). oc2, the coverage of adsorbed ethene during the reaction, which is normalized by the number of the surface Pt atoms measured by hydrogen adsorption experiment. Reactions were carried out at 247 K, and the base pressure of the reactants is 1.3 kPa.
0.32 0.31
0.2}
0.4
0 0
1
2
3
4
5
2
3
6
5
6
Na /Pt Figure 5. Initial rate for ethane formation on Na/Pt/Si02 as a function of the amount of Na. Reaction temperature, 247 K PC,,WH~ = 3.0 kpa; PD2 = 1.3 kPa. The rates were normalized by the number of surface Pt
6
Na / Pt Figure 4.
1
fd for Na/Pt/Si02 as a function of Na loading.
atoms.
with all the substituents except for CF,; the following equation is fitted:
+
In r = - 6 . 4 ~ ~ 1.4 (except CF,) (4) The typical XANES spectra of the Pt L3 edge of Pt/Si02 in the presence of CH2=CHX (X = H, CH,, CN, COCH,, CF,) are illustrated in Figure 3. The value offd derived from the XANES spectra are listed in Table I, together with up. The electron-withdrawing character of total adsorbed molecules can be estimated by the difference (Afd),fd(CH2=CHX) -fd(none), which is also listed in Table I. fd was observed to correlate with upexcept for CH2=CHCF3. As for CH2=CHCF3,fd was much smaller than expected from up, which may be related to its extremely high reaction rate as shown in Figure 2. fd for Pt in Na/Pt/Si02 under vacuum as a function of Na loading is plotted in Figure 4. The figure shows thatfd decreases monotonously with an increase of Na quantity. The particle size of Pt was enlarged in the presence of Na;" Le., the dispersion of Pt particles measured by hydrogen adsorption was 0.31 for Pt/Si02 and 0.13 for Na/Pt/Si02 with Na/(Na + Pt) = 0.43 0.86. The density of the unoccupied d state has been demonstrated to depend on the particle size of Pt in Pt/Si02 catalyst.'*2 Table 11 shows the variation offd and hT with the particle size of Pt/Si02. The d electron density decreased with the particle size, but the difference was small for H I M < 0.3. Thus, it is concluded that the effect of the particle size on the value offd is small as H I M I0.30for the present Na/F%/Si02 catalysts. Hence, the change offd in Figure 4 is ascribed mainly to the electron donation from sodium to platinum. The kinetic parameters for the reaction and the amount of molecules adsorbed on the catalysts are summarized in Table 111. The rates (r) of ethane formation on Na/Pt/Si02 under identical conditions are plotted against the Na quantity in Figure 5 . The reaction was promoted by the addition of Na. The rate was expressed by the equation
-
r = ~ D ~ P D ~+ /( KEPE) I
where PD2and (17) Yoshitake,
f E
(5)
are the partial pressures of D2 and ethene,
H.;Iwasawa, Y. J . Cotol., in press.
00.1 2/
-0.1
1
-0.21
-0.31 0
,
,
,
,
,
,
1
2
3
4
5
6
Na/Pt Figure 6. fd for Na/Pt/SiO, under ethene of 1.5 kPa as a function of Na loading.
/
06t
i
0 2 J 00-00 0
1
2
3
4
5
6
Na/Pt Figure 7. Number of electrons eliminated per an adsorbed ethene molecule (AhTo) and di-a-ethene (Ah,',) as a function of Na loading.
respectively, and kD2and KEare constants. The values in Table 111 were obtained through least-squres fitting to eq 5 . The activation energies changed slightly from 33 kJ-mol-' for Pt/Si02 to 41 W-mol-' for Na/Pt/Si02, but they were independent of Na loading. The adsorption amount of ethene was measured at certain
XANES of Pt Catalyst in Reaction Conditions
J
3100
BOO 2900
2800
wavenumber / cm
Figure 8. IR spectra of ethene adsorbed on Na/Pt/Si02. Na/Pt = (a) 0.0, (b) 0.19, (c) 0.75, (d) 1.5, (e) 6.1.
intervals during the reaction to give e,-, the coverage of ethene, by the extrapolation to the value at the initiation of the reaction. The coverage also decreased according to the Na amount. To examine the relationship of catalytic performance with the electron density of unoccupied d state, fd was measured in the presence of ambient ethene similar to catalytic reaction conditions. The values are plotted in Figure 6, wherefd is illustrated as a function of the amount of Na contained in the catalysts. fd was decreased once and then increased with the Na loading in contrast to Figure 4. Figure 6 is the effect of total ethene adsorbed on the Pt surface. To see the effects of each adsorbed ethene molecule onfd,fd in Figure 6 was converted into the amount of electrons removed per ethene molecule adsorbed (AhTo) in Figure 7. AhTO was obtained by dividing the difference offd in Figure 4 and Figure 6 by Bc2. AhTo was found to be proportional to Na loading. Infrared spectra were observed to attribute the type of adsorbed ethene which withdraws electrons of Pt during the reaction on Na/Pt/Si02. The charts of the C-H stretching peaks for the adsorbed ethene are shown in Figure 8. The bands at 3078-3079, 3025-3023, and 2965-2964 cm-l are attributed to r-ethene on Pt, and the bands at 2888-2863 cm-’ are assigned to di-u-ethene on Pt.I8*l9 A small peak of 2803 cm-I for Pt/Si02 has been assigned to be ethylidyne. The bands for di-u-ethene markedly shifted from 2888 to 2863 cm-I by Na loading, whereas the peak positions for r-ethene did not change. Discussion (i) The Linear Energy Relationship and Unoccupied d8tate Density. The linear free energy relationship (LFER) has been found in many chemical reactions of organic compounds. One of the indicators widely used is Hammett’s u. LFER has been reported in the dissociation reaction of olefin ligands in mononuclear organometallic complexes:2*22 L.M-~,
-
L,M+ =\ X
According to these studies, a negative correlation between up and the dissociation constant (&) was found. It suggests that the (IS) Beebe, T. P., Jr.; Yates, J. T., Jr. J . Phys. Chem. 1987, 91, 254. (19) Mohsin, S.B.; Trenary, M.; Robota, H. J. J . Phys. Chem. 1988.92, 5229. (20) Tolman, C. A. J . Am. Chem. SOC.1974, 96, 2780. (21) Yamamoto, T.; Yamamoto, A.; Ikeda, S. J . Am. Chem. SOC.1971, 93, 3360. (22) Cramer, R. J . Am. Chem. SOC.1964,86, 217.
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7371
strength of the chemical bond between C - C and group 8 metals is mainly controlled by the ?r backdonation from the d state (virtual HOMO) of the metal to LUMO of the ligand. All the CHI= CHX compounds used in the present study have been proved to show the LFER without any exception in the N i ( P ( o - t ~ l y l ) ~ ) ~ system?O in which Kd becomes smaller as up of x becomes larger. Thus, it is suggested that the adsorption of CH,=CHX on Pt becomes strong with the larger up and the deuterogenation rate should be damped, because the rate of ethene deuterogenation, represented by eq 5 , is based on the competitive adsorption of Dz and CHz=CH2 and the rate-determining adsorption of D2. However, several questions arise: (1) Is LFER really valid for heterogeneous hydrogenation systems? (2) Does the d state of Pt change during the reaction? (3) Does the change in the d state affect the reaction mechanism for ethane formation? How about the activation energy and the frequency factor? For the rate of the deuterogenation, an LFER is shown in Figure 2. This is consistent with the rate expression 5. Consequently, it is rational to monitor the d state under the reaction conditions in the presence of CH2=CHX. The density of the unoccupied d state of Pt in Pt/Si02 was changed in relation to the electron-withdrawing character of substituents (X) of CHz=CHX (up) except for X = CF3 as demonstrated in Table I. The larger the up of the substituent, the larger the unoccupied d state becomes. Nevertheless, this d-electronic change brought no electronic modification to the step of D2 adsorption as the rate-determining step. In fact, Figure 1 showed the same activation energy for all CH2=CHX. The rate variation with the kind of CH2=CHX results from the alternation of the preexponential factor in the rate constant (kD2), The rate for the deuterogenation of CH2=CHCF3 is much larger than that expected from the line in LFER of Figure 2. A small change in unoccupied d density, Ah, observed on the adsorption of CH2=CHCF3 in Table I demonstrates a weak adsorption of the molecule on platinum. It has been reported that the mode of adsorption on Pt( 111) and (100) is not dissociative,u” excluding the contribution of self-hydrogenation to alkane formation and the direct interaction between fluorine and platinum. In fact, no formation of CH3CHzCF3by CH2=CHCF3alone was observed with Pt/Si02 catalyst,and no hydrogenolysis was observed under the reaction condition. up has been demonstrated to have a good correlation with the associative constant in mononuclear complexes.20 On the other hand, CHz=CHX adsorbs ~ Jmay ~ on multimetal sites at Pt( 11 1) and (100) s u r f a c e ~ . ~up not always reflect the strength of the electron-withdrawing character of substituents (X) at metal surfaces. The effective value of up for CF3 on Pt surface is evaluated to be -0.28 from Figure 2. (ii) The Donation by NazO and the Distribution of Electrons during the Reaction. Another way to modify the interaction between the C=C bond and Pt is to add an alkali metal to the surface. The electron-withdrawing group makes the LUMO of CH2=CHX low, and the electron-donating additive makes the Fermi level high. These may strengthen the chemical bond between the C=C bond and Pt to result in the suppression of D2 adsorption. The behavior offd in Figure 4 is explained by electron donation from added NazO, which fills the unoccupied d state of Pt as a function of the amount of Na. This tendency is consistent with our previous result for Pt 4f7/2I7in X-ray photoemission spectroscopy (XPS), where the decreasing binding energy shift as a function of Na amount was observed. The unoccupied d state of Pt in Na/Pt/SiO, varied from 0.32 (Na/Pt = 0.0) to 0.22 (Na/Pt = 6.1) in Figure 4. The values of hT for Pt/Si02 with different particle sizes in Table I1 varied from 0.35 to 0.31, and hT for 0.7% Pt/SiO, prepared by an ion-exchange method was reported to be 0.37.9 Thus, the degree of the change in hT by Na addition is comparable to that by (23) Clarke, T. A,; Gay, 1. D.; Law, B.;Mason, R. Faraday Discuss. Chem. SOC.1976, 60, 119. (24) Clarke, T. A.; Gay, 1. D.; Mason, R. Chem. Phys. Lett. 1974, I , 562.
7372 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 particle size effect, but the values for small Pt particles are larger than h, (~0.30).whereas the Na additives make the value smaller than h,. An increase of the activity for ethane formation (Figure 5) and the continuous decrease of KE (Table 111) indicate that the filling d states suppress the adsorption of CH2==CH2. The adsorption of ethene on a Pt single crystal has been suggested to be weakened by alkali metal in the The decrease of koz by the small amount of Na may be due to the increase of activation energy and the site blocking of Na20 on the Pt surface. In contrast to KE,kD2increased with an increase of Na quantity in Table 111, while the activation energy was constant. These results show that the adsorption of ethene on platinum in a working state of D2 + CH2=CH2reaction is suppressed when the density of the d state under vacuum becomes larger by Na and, as a result, the rate of D2dissociation is increased. This may be contradictory to the HOMO-LUMO interaction picture2’ for the adsorption as discussed above. Thus, it is natural to have questions about the density of the unoccupied d state of Pt and the strength of the adsorption of ethene in a working state of the catalyst. Most of the charge gained by Pt was found to be lost under ambient CH2==CH2 gas; that is, the extra charge from N a 2 0 is almost extracted by ethene adsorbed OR Pt. Little change offd under reaction conditions can claim almost no change in the activation energy in Table 111. Figure 6 illustrates the whole state of platinum particles, which is proper for discussing the electronic effect on D2 adsorption, but does not show the microscopic state around active sites and adsorbed ethene. Thus& in Figure 6 was converted into the amount of electrons extracted per molecule. The number of electrons in the d state extracted per adsorbed ethene increased linearly up to 0.76 at Na/Pt = 6.1 as shown in Figure 7. Thus, it is suggested that ethene molecules play a role of electron buffer at the Pt surface during the deuterogenation of ethene on Na/Pt/Si02 by the increase in the number of electrons accepted by one ethene molecule adsorbed rather than the increase in the amount of adsorbed ethene. The initial sharp drop offd by Na addition in Figures 4 and 6 is attributed to the decrease of surface Pt atoms (increase of the particle size) in the presence of Na, but AhT for Pt/SiO, is also on the same line as those for Na/Pt/Si02 in Figure 7. We have already reported that the reaction intermediate for ethane formation in D2 CH2=CH2 reaction on Na/Pt/SiO, catalyst is di-u-ethene, while that for hydrogen exchange during the reaction is x-ethene. The former reaction occurs on the far site on Pt surface and the latter on the neighbor site around Na2O.I7 Figure 8 shows the peak position shift downward by 18-25 cm-’ for di-a-ethene, which is suggested to result from the
+
(25) Windham, R. G.; Kocl, B. E. J . Phys. Chem. 1988,92, 2862. (26) Zhou, X.-L.; Zhu, X.-Y.; White, J. M. SurJ Scf. 1988, 193, 387. (27) Fleming, 1. Fronrfer Orbfrulsand Organic Chemicul Reocrionr; Wiley & Sons: London, 1976; Chapter 2.
Yoshitake and Iwasawa acceptance of the extra charge donated by Na20. On the contrary, x species show no peak shift. This implies r species remain in the same environment in the presence of Na20. x- and di-aethenes are the main adsorbates as intermediates on the noble metals at room temperatureVa The ratio, di-a/(r + di-a), on Na/Pt/Si02 was estimated to be 0.7 by the analysis of deuterated ethane and ethene produced by the reactions of the two kinds of adsorbates with D2. The degree of the electrons eliminated per di-a-ethene (AhT’,) is also shown in the scale on the right-hand side of Figure 7, assuming that no r-ethene accepts the extra charge of Pt. The maximum value of A ~ T O , is 1.1 (Na/Pt = 6.1), and it is suggested that the adsorbed intermediate for ethane formation on Na/Pt/Si02 is di-a-CH2CH2* where n varies from 0.1 to 1.1. Conclusion 1. In the adsorption of CH2=CHX (X = H, CH3, COCH3,
CF3, CN) on Pt/Si02 catalyst, fd of Pt was correlated with Hammett’s upof X except for CF3,Le., the larger the up the larger the density of the unoccupied d state becomes. 2. The reaction rates for the deuterogenation of CH2=CHX including CH2=CHCF3were inversely correlated with the density of the unoccupied d state determined from the white lines in XANES spectra at pt L2and L3 edges. 3. The addition of N a 2 0 to Pt/Si02 increased the activation energy a little but promoted ethene deuterogenation as a function of Na quantity. 4. The promotion of the rate is due to the decrease of the amount of adsorbed ethene and hence due to the increase of the preexponential factor of the rate constant for D2 dissociative adsorption which is rate-determining. 5. The Na20 additives decreased the unoccupied d-state density of Pt under vacuum. N a 2 0 donates electrons to the d states of Pt. 6. However, under the reaction conditions, most of the electrons of the d state donated from Na10 moved to ethene adsorbates. 7. The number of d electrons abstracted per adsorbed ethene molecule was proportional to the Na quantity in Na/pt/Si02 catalysts. 8. The acceptor of the electrons was suggested to be di-a-ethene, which is a reaction intermediate for ethane formation.
Acknowledgment. We are grateful to Prof. M. Nomura and the staff of Photon Factory for their help in the XANES measurements (Proposal 89-007). We thank Japan Halon Inc. for the offer of 3,3,3-trifluoropropene. Registry No. Na, 7440-23-5; CH2==CHX (X = H), 74-85-1; CH2= CHX (X = CH3), 115-07-1; CHI==CHX (X = CN), 107-13-1; CH2= CHX (X = COCH,), 78-94-4; C H 2 = C H X (X = CF,), 677-21-4; N a 2 0 , 131 3-59-3. (28) Sheppard, N. Annu. Rev. Phys. Chem. 1988, 39, 589.
